1. Field of the Invention
This invention relates to optical systems of reflecting type and image pickup apparatuses using the same and, more particularly, to such optical systems which, using an optical element of many reflecting surfaces, form an object image on a predetermined plane. Still more particularly, this invention relates to improvements of the compact form of the entirety of the optical system suited to video cameras, still cameras or copying machines.
2. Description of the Related Art
There have been many previous proposals for utilizing the reflecting surfaces of convex and concave mirrors in the optical system for an image pickup apparatus. FIG. 24 schematically shows a so-called mirror optical system composed of one concave mirror and one convex mirror.
In the so-called mirror optical system of FIG. 24, an axial beam 104 coming from an object is reflected from the concave mirror 101. While being converged, it goes toward the object side and is then reflected by the convex mirror 102 to form an image on an image plane 103.
This mirror optical system is based on the configuration of the so-called Cassegrainian reflecting telescope. The aim of adopting it is to shorten the total length of the entire system as the equivalent telephoto system which is constructed with refracting surfaces or lenses alone has a long total length. To this purpose, the optical path is folded twice by using two reflecting surfaces arranged in confronting relation.
Even for the objective lens systems of telescopes, besides the Cassegrainian type, there are known, from a similar reason, a large number of forms with the use of a plurality of reflecting mirrors in shortening the total length of the optical system.
Like this, in a case where a photographic lens would take a long total length, it has been the common practice to employ reflecting mirrors instead of some of the lens members. By folding the optical path to good efficiency, a compact mirror optical system is obtained.
However, the Cassegrainian reflecting telescopes and like mirror optical systems generally suffer a problem due to the vignetting effect by the convex mirror 102, as the object light beam is partly mutilated.
This problem will exist so long as the convex mirror 102 is laid at the central passage of the object beam 104.
To solve this problem, the reflecting mirror may be put decentered, thus avoiding obstruction of the passage of the object beam 104 by the unintegrated part of the optical system. In other words, the principal ray 106 of the light beam is dislocated away from an optical axis 105. Such an optical system, too, has previously been proposed.
FIG. 25 is a schematic diagram of a mirror optical system disclosed in U.S. Pat. No. 3,674,334, wherein the problem of mutilation described above is solved in such a way that the reflecting mirrors to be used are rotationally symmetric with respect to the optical axis and partly cut off.
The mirror optical system of FIG. 25 comprises, in order of passage of the light beam, a concave mirror 111, a convex mirror 113 and a concave mirror 112, which, when in the prototype design, are, as shown by the double dot-and-dash lines, the complete reflecting surfaces of rotational symmetry with respect to the optical axis 114. Of these, the concave mirror 111 is used only in the upper half on the paper of the drawing with respect to the optical axis 114, the convex mirror 113 only in the lower half and the concave mirror 112 only in a lower marginal portion, thereby bringing the principal ray 116 of the object beam 115 into dislocation away from the optical axis 114. The optical system is thus made free from the mutilation of the object beam 115.
FIG. 26 shows another mirror optical system which is disclosed in U.S. Pat. No. 5,063,586. The reflecting mirrors have their central axes made themselves to decenter from the optical axis. As a result, the principal ray of the object beam is dislocated from the optical axis, thus solving the above-described problem.
Referring to FIG. 26, assume that the perpendicular line 127 to the object plane 121 is an optical axis. With a convex mirror 122, a concave mirror 123, a convex mirror 124 and a concave mirror 125 in order of passage of the light beam, it is then proven that the centers of their reflecting areas do not fall on the optical axis 127 and that their central axes (the lines connecting those centers with the respective centers of curvature of the reflecting surfaces) 122a, 123a, 124a and 125a are decentered from the optical axis 127. In connection with this figure, the decentering amount and the radius of curvatures of every one surface are appropriately determined to prevent the object beam 128 from being mutilated by the other mirrors. Thus, an object image is formed on a focal plane 126 with high efficiency.
Besides these, U.S. Pat. Nos. 4,737,021 and 4,265,510 even disclose similar systems freed from the vignetting effect either by using certain portions of the reflecting mirrors of revolution symmetry about the optical axis or by decentering the central axes themselves of the reflecting mirrors from the optical axis.
These reflecting type photographic optical systems, because they have a great number of constituent parts, require highly precise assembly of the individual optical parts to insure satisfactory optical performance. In particular, because the tolerance for the relative positions of the reflecting mirrors is severe, later adjustment of the position and angle of orientation of each reflecting mirror is indispensable.
To solve this problem, one of the proposed methods is to construct the mirror system in the form of, for example, a block, thus avoiding the error which would otherwise result from the stepwise incorporation of the optical parts when in assembling.
It has been known to provide one block with a large number of reflecting surfaces. For example, the viewfinder systems employ optical prisms such as pentagonal roof prisms or Porro prisms.
These prisms are made by the molding techniques to unify the plurality of reflecting surfaces. Therefore, all the reflecting surfaces take their relative positions in so much good accuracy as to obviate the necessity of the positions of the reflecting surfaces relative to one another. However, the main function of these prisms is to change the direction of travel of light for the purpose of inverting the image. Every reflecting surface is, therefore, made to be a flat surface.
For the counterpart to this, there is also known an optical system by giving curvature to the reflecting surface of the prism.
FIG. 27 is a schematic diagram of the main parts of an observing optical system disclosed in U.S. Pat. No. 4,775,217. This optical system is used for observing the external field or landscape and, at the same time, presenting an information display of data and icons in overlapping relation on the landscape.
The rays of light 145 radiating from the information display device 141 are reflected from a surface 142, going to the object side until they arrive at a half-mirror 143 of concave curvature. The reflected ones of the light rays 145 from the half-mirror 143 are nearly collimated by the refractive power of the concave surface 143, and refract in crossing the surface 142, reaching the eye 144 of the observer. The observer views an enlarged virtual image of the displayed data or icons.
Meanwhile, a light beam 146 from an object enters at a surface 147 which is nearly parallel with the reflecting surface 142, and is refracted by it and arrives at the concave surface 143. Since this surface 143 is coated with a half-permeable layer by the vacuum evaporation technique, part of the light beam 146 penetrates the concave surface 143 and refracts in crossing the surface 142, entering the pupil 144 of the observer. So, the observer views the display image in overlapping relation on the external field or landscape.
FIG. 28 is a schematic diagram of the main parts of another observing optical system disclosed in Japanese Laid-Open Patent Application No. Hei 2-297516. This optical system, too, is used for viewing the external field or landscape and, at the same time, noticing the information on the display device in overlapping relation.
In this system, a light beam 154 from an information display 150 enters a prism Pa at a flat surface 157 and is incident on a paraboloidal reflecting surface 151. Being reflected from this surface 151, the display light beam 154 becomes a converging beam. Before the display light beam 154 forms an image on a focal plane 156, three total reflections occur as the beam 154 travels between two parallel flat surfaces 157 and 158 of the prism Pa. A thinning of the entirety of the optical system is thus achieved.
From the focal plane 156, the display light beam 154 exits as a diverging beam and, while repeating total reflection from the flat surfaces 157 and 158, goes on until it is incident on a paraboloidal surface 152. Since this surface 152 is a half-mirror, the beam 154 is reflected and, at the same time, undergoes its refractive power, forming an enlarged virtual image of the display and becoming a nearly parallel beam. After having penetrated the surface 157, the beam 154 enters the pupil 153 of the observer. Thus, the observer looks at the display image on the background of the external field or landscape.
Meanwhile, an object light beam 155 from the external field passes through a flat surface 158b constituting a prism Pb, then penetrates the paraboloidal half-mirror 152 and then exits from the surface 157, reaching the eye 153 of the observer. So, the observer views the external field or landscape with the display image overlapping thereon.
Further, an optical element can be used in the reflecting surface of the prism. This is exemplified as disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 5-12704 and Hei 6-139612 as applied to the optical head for photo-pickup. Such a head receives the light from a semiconductor laser, then reflects it from the Fresnel surface or hologram surface to form an image on a disk, and then conducts the reflected light from the disk to a detector.
The mirror optical systems of the U.S. Pat. Nos. 3,674,334, 5,063,586 and 4,265,510 mentioned before have a common feature that all the reflecting mirrors are made decentered by respective different amounts to one another. Hence, the mounting mechanism for the reflecting mirrors becomes very complicated in structure. It is also very difficult to secure the acceptable mount tolerance.
It should be also noted that known reflecting-type photographic optical systems are adapted for application to the so-called telephoto type of lens systems as this type has a long total length and a small field angle. To attain a photographic optical system handling field angles from the standard lens to the wide-angle lens, which require an increasing number of reflecting surfaces for correcting aberrations, the parts must be manufactured even more precisely and assembled with even severer a tolerance. Therefore, production costs rise. Otherwise, the size of the entire system tends to increase largely.
Also, the observing optical systems of the U.S. Pat. No. 4,775,217 and the Japanese Laid-Open Patent Application No. Hei 2-297516 mentioned before each have an aim chiefly to produce the pupil image forming function such that, as the information display is positioned remotely of the observer's eye, the light is conducted with high efficiency to the pupil of the observer. Another chief aim is to change the direction of travel of the light. Concerning the positive use of the curvature-imparted reflecting surface in correcting aberrations, therefore, no technical ideas are directly disclosed.
Also, the optical systems for photo-pickup of the Japanese Laid-Open Patent Applications Nos. Hei 5-12704 and Hei 6-139612 mentioned before each limit its use in the detecting purpose. Therefore, these systems are unable to satisfy the imaging performance for photographic optical systems and particularly image. pickup apparatus using CCD or like area type image sensor.